Catalytic processes for the conversion of hydrocarbons using platinum group metals and a catalyst support are well known and extensively used. One such process is reforming and another is olefin production. Eventually the catalysts used in these processes become deactivated for one or more reasons. Where the accumulation of coke deposits causes the deactivation, reconditioning of the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from catalyst by contact of the coke containing catalyst at high temperature with an oxygen-containing gas to combust and remove the coke in a regeneration process. These processes can be carried out in-situ or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a reaction zone and for coke removal in a regeneration zone are well known.
In order to combust coke in a typical regeneration zone, a recycle gas is continuously circulated to the combustion zone of a regeneration section and a flue gas containing the by-products of coke combustion, oxygen and water is continually withdrawn. Coke combustion is controlled by recycling a low oxygen concentration gas into contact with the coke-bearing catalyst particles. The flue gas/recycle gas is continuously circulated through the catalyst particles in a recycle gas loop. A small stream of combustion gas is added to the recycle gas to replace oxygen consumed in the combustion of coke and a small amount of flue gas is vented off to allow for the addition of the combustion gas. The steady addition of combustion gas and the venting of flue gas establishes a steady state condition that produces a nearly constant concentration of water and oxygen in the recycle gas.
In prior art regeneration methods, after the combustion zone, the metal-containing catalyst particles pass downwardly to a subadjacent halogenation zone. Chlorine or other halogen-containing gas circulates through the halogenation zone. Contact with the halogenation gas redisperses the platinum group metal on the catalyst particles.
From the halogenation zone, catalyst particles descend into a subadjacent drying zone. A heated gas contacts the catalyst particles and drives moisture from the catalyst. Typically, air or an oxygen-containing gas enters the drying zone as the drying medium and passes upward through the halogenation zones.
In continuous or semi-continuous regeneration processes, coke laden particles are at least periodically added and withdrawn from a bed of catalyst in which the coke is combusted. Regions of intense burning that extend through portions of the catalyst bed develop as the coke is combusted.
The regenerator holds catalyst undergoing regeneration in one or more catalyst beds. Catalyst beds usually take on one of two fluid-flow configurations, a radial flow arrangement, or a vertical flow arrangement. In either type of bed, catalyst particles must downwardly pass freely through the bed to transfer catalyst and obtain a continuous or semi-continuous regeneration. Radial or vertical gas flow through the bed can interfere with the free movement of catalyst particles and hinder the transfer of particles through the bed during the regeneration process.
Experience has shown that horizontal flow of reactants, in particular, through a radial bed of catalyst, can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on horizontal flow reactor designs. Catalyst pinning occurs when the frictional forces between catalyst pills that resist the downward movement of the catalyst pills are greater than the gravitational forces acting to pull the catalyst pills downward. The frictional forces occur when the horizontal flow vapor passes through the catalyst bed. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from its outer boundary by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and the outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and increase catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward through the bed. With radially outward flow, void blowing occurs when the frictional forces between the catalyst pills are greater than the gravitational forces, or in other words, at about the same time as pinning would occur with a radially inward flow. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems. Catalyst fines can also accumulate in the narrow openings of the screen surfaces used to retain the catalyst particles thereby plugging these surfaces and requiring a shut-down of the equipment to remove catalyst fines. Catalyst fines are usually more abrasive than the larger catalyst particles and thereby contribute to greater erosion of the process equipment. Finally, the catalyst in many of these hydrocarbon conversion processes is a valuable commodity and the generation and removal of catalyst fines imposes a direct catalyst cost on the operation of the system.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. In vertical or axial flow beds, the upward flow of gases there through can also pose problems of catalyst hang-ups and fluidization. As gas passes upwardly through a bed of particles at low velocity, it migrates through the particles without changing the density of the catalyst bed. As velocity increases, the flow of gas creates drag forces that lift the particles. When these drag forces exceed the weight of the catalyst, frictional forces between the particles drop to zero and the bed approaches a fluidized state. With continued increases in gas velocity, the particles experience lift that tends to transport the particles upwardly out of the bed. Some upward gas flow through the bed is useful since it promotes the movement of catalyst particles by reducing interparticle friction so that the particles flow more as a fluid. However, increasing upward drag forces can suspend catalyst particles and prevent downward catalyst movement. As a result, the upward gas velocity through an axial bed of catalyst particles must be limited to permit gravity flow removal of the catalyst particles.
Accordingly, the problems of hang-up catalyst pinning and void blowing limit the gas velocity through the catalyst beds in the regeneration process. The combustion of a fixed quantity of coke in the regeneration process burns a proportional quantity of oxygen. Supplying the stoichiometric oxygen requirements for the coke combustion demands the circulation of sufficient oxygen through the catalyst bed. Higher oxygen demands during transitory periods of heavy coke burning require a temporary increase in the oxygen supply to the combustion zone of the regeneration section. The oxygen delivery to the combustion zone varies with the gas velocity and oxygen concentration of the gas stream. Since gravity removal of catalyst particles may limit the gas flow velocity, the only ways to increase coke combustion within the catalyst bed is to increase the oxygen concentration or the size of the bed. Physical dimensions of the regeneration section that determine the size of the bed are not easily changed for temporary fluctuations in the process. Charging more air or other oxygen-containing gas to the combustion zone increases the available oxygen for coke combustion. Recycle gas normally has a low oxygen concentration of about 0.5 to 2 mole percent.
However, under upset conditions the catalyst particles may become heavily coked in a hydrocarbon conversion process and then the catalyst particles may not be adequately and fully regenerated during conventional regeneration procedures. In order to recover the activity of heavily coked catalyst particles, it is necessary to contact the coked catalyst particles with a contacting gas containing increased concentrations of oxygen and a sufficient contacting gas flow rate to ensure that the resulting heat of combustion is successfully removed to prevent any possible permanent catalyst damage.
In order to maintain the oxygen concentration in the primary combustion zone of the regenerator at less than about 1.4 mole percent and also to purge halide compounds, water and heat of combustion from the regenerator, nitrogen is added to the regeneration zone gas circulation loop.